I’m planning to install Windows 7 on a second hard drive. On the first there is a working Antergos install that should not be damaged. If possible I’d prefer to avoid a Grub2 restore too.
The computer is an old desktop with bios (no UEFI) and I can plug in and out the hard disks separately.

So Windows should be installed on /dev/sdb, using entire /dev/sdb.
Can I simply plug out /dev/sda (to prevent windows from overwriting Grub2 on dev/sda), install windows on /dev/sdb, plug out /dev/sdb after successful installing windows, plug in /dev/sda boot Antergos and do simply

sudo update-grub

Will this work ? Can I avoid this way restoring Grub2 ?
Or is it necessary to have both hard disks plugged in during installation und restoring Grub2 afterwards ?
Can I use super grub disk for restoring Grub2 if necessary ?

Didn’t install Windows for such a long time, but my wife needs a fallback machine for her windows notebook.

Install windows as normal, just make sure you install it on the correct drive, and reboot into antergos, then update grub. It should identify your windows installation and automatically create an entry. If it does not automatically find windows you can just manually create an entry.